U.S. patent application number 12/909780 was filed with the patent office on 2011-02-17 for method of performing uplink synchronization in random access procedure.
Invention is credited to Sung Duck Chun, Young Dae Lee, Sung Jun PARK, Seung June Yi.
Application Number | 20110038361 12/909780 |
Document ID | / |
Family ID | 40720484 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110038361 |
Kind Code |
A1 |
PARK; Sung Jun ; et
al. |
February 17, 2011 |
METHOD OF PERFORMING UPLINK SYNCHRONIZATION IN RANDOM ACCESS
PROCEDURE
Abstract
A method and device for performing a contention based random
access procedure by a mobile communication terminal in
communication with a base station. The method according to an
embodiment includes transmitting a random access preamble message
to the base station; receiving a random access response from the
base station, the random access response including a timing advance
command; determining a status of a mobile communication terminal
time alignment timer; and ignoring the timing advance command if
the mobile communication terminal time alignment timer is
determined to be running in the determining step.
Inventors: |
PARK; Sung Jun; (Anyang-si,
KR) ; Yi; Seung June; (Anyang-si, KR) ; Lee;
Young Dae; (Anyang-si, KR) ; Chun; Sung Duck;
(Anyang-si, KR) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
40720484 |
Appl. No.: |
12/909780 |
Filed: |
October 21, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12392654 |
Feb 25, 2009 |
7843895 |
|
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12909780 |
|
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Current U.S.
Class: |
370/350 |
Current CPC
Class: |
H04W 74/002 20130101;
H04W 72/0446 20130101; H04W 56/0005 20130101; H04W 74/0833
20130101; H04J 11/003 20130101; H04W 88/16 20130101; H04J 2011/0096
20130101; H04W 56/0045 20130101; H04B 7/2681 20130101; H04W 72/0413
20130101 |
Class at
Publication: |
370/350 |
International
Class: |
H04J 3/06 20060101
H04J003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2008 |
KR |
10-2008-0023807 |
Claims
1. A method of performing a random access procedure by a mobile
terminal in communication with a base station, comprising:
transmitting a random access preamble to the base station;
receiving from the base station a random access response as a
response to the random access preamble, the random access response
including an uplink resource assignment and time alignment
information; determining whether an uplink is synchronized or not
when the random access procedure is contention based; and if the
uplink is not synchronized, applying the time alignment
information.
2. The method of claim 1, further comprising: if the uplink is
synchronized, ignoring the time alignment information.
3. The method of claim 2, further comprising: initiating contention
resolution with another mobile terminal trying the random access
procedure by transmitting a scheduled message using the uplink
resource assignment to the base station.
4. The method of claim 1, wherein the random access preamble is
randomly selected from a plurality of available random access
preambles when the random access procedure is contention based.
5. The method of claim 1, further comprising: receiving a message
from the base station before transmitting the random access
preamble, the message comprising second time alignment information;
and applying the second time alignment information for uplink
synchronization.
6. The method of claim 5, wherein the message is not a random
access response.
7. The method of claim 1, further comprising: applying the time
alignment information when the random access procedure is
non-contention based.
8. A mobile terminal configured to perform a random access
procedure with a base station, the mobile terminal comprising: a
radio frequency (RF) unit; and a processor operatively connected to
the display and configured for: transmitting a random access
preamble to the base station; receiving from the base station a
random access response as a response to the random access preamble,
the random access response including an uplink resource assignment
and time alignment information; determining whether an uplink is
synchronized or not when the random access procedure is contention
based; and if the uplink is not synchronized, applying the time
alignment information.
9. The mobile terminal of claim 8, wherein the processor is
configured for ignoring the time alignment information if the
uplink is synchronized.
10. The mobile terminal of claim 9, wherein the processor is
configured for initiating contention resolution with another mobile
terminal trying the random access procedure by transmitting a
scheduled message using the uplink resource assignment to the base
station when the uplink is synchronized.
11. The mobile terminal of claim 8, wherein the processor is
configured for selecting randomly the random access preamble from a
plurality of available random access preambles when the random
access procedure is contention based.
12. The mobile terminal of claim 8, wherein the processor is
configured for: receiving a message from the base station before
transmitting the random access preamble, the message comprising
second time alignment information; and applying the second time
alignment information for uplink synchronization.
13. The mobile terminal of claim 8, wherein the processor is
configured for applying the time alignment information when the
random access procedure is non-contention based.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is continuation of co-pending U.S.
application Ser. No. 12/392,654 filed on Feb. 25, 2009, which
claims priority to Korean Patent Application No. 2008-0023807 filed
on Mar. 14, 2008. The entire contents of each of these applications
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to wireless communications,
and more particularly, to a method of performing uplink
synchronization through a random access procedure in a wireless
communication system.
[0004] 2. Discussion of the Background Art
[0005] Third generation partnership project (3GPP) mobile
communication systems based on a wideband code division multiple
access (WCDMA) radio access technology are widely spread all over
the world. High-speed downlink packet access (HSDPA) that can be
defined as a first evolutionary stage of WCDMA provides 3GPP with a
radio access technique that is highly competitive in the mid-term
future. However, since requirements and expectations of users and
service providers are continuously increased and developments of
competing radio access techniques are continuously in progress, new
technical evolutions in 3GPP are required to secure competitiveness
in the future. Reduction of cost per bit, increase of service
availability, flexible use of frequency bands, simple structure and
open interface, proper power consumption of a user equipment (UE),
and the like are defined as requirements.
[0006] In general, there are one or more cells within the coverage
of a base station (BS). One cell may include a plurality of UEs. A
UE generally performs a random access procedure to access a
network. The random access procedure is performed for various
purposes, for example, for uplink synchronization between the UE
and the network, uplink radio resource allocation request, etc.
[0007] The random access procedure starts when the UE transmits a
random access preamble. The UE randomly selects one random access
preamble from predetermined 64 random access preambles, and
transmits the selected random access preamble. Although one
preamble is randomly selected from 64 random access preambles, two
or more UE can simultaneously perform the random access procedure
by using the same random access preamble. This is called a
contention-based random access procedure. On the contrary, when a
dedicated random access preamble is assigned to each UE, it is
called a non-contention based random access procedure.
[0008] One of the purposes of performing the random access
procedure is to perform uplink synchronization. When collision
occurs between UEs in the contention based random access procedure,
whether collision occurs or not cannot be known until contention
resolution is achieved between the BS and the UE. Therefore, even
if collision occurs, the UE may perform uplink synchronization by
using time alignment information transmitted from the BS, resulting
in incorrect synchronization. The incorrect uplink synchronization
performed by the UE may act as interference to another UE, which
may lead to deterioration in reliability of a communication
system.
SUMMARY OF THE INVENTION
[0009] The present invention provides a method of preventing a user
equipment from performing incorrect uplink synchronization when the
user equipment fails in contention resolution in a random access
procedure.
[0010] The present invention also provides a method of preventing
one user equipment from causing interference to another user
equipment when synchronization correction is not properly achieved
in a random access procedure.
[0011] In an aspect of the invention, there is a method of
performing a contention based random access procedure by a mobile
communication terminal in communication with a base station. The
method includes transmitting a random access preamble message to
the base station; receiving a random access response from the base
station, the random access response including a timing advance
command; determining a status of a mobile communication terminal
time alignment timer; and ignoring the timing advance command if
the mobile communication terminal time alignment timer is
determined to be running in the determining step.
[0012] In an aspect of the invention, the timing advance command is
configured to adjust, start or restart the mobile communication
terminal time alignment timer.
[0013] In an aspect of the invention, the method includes
initiating contention resolution by transmitting a scheduled
message to the base station.
[0014] In an aspect of the invention, the method includes applying
the timing advance command to the mobile communication terminal
time alignment timer if the mobile communication terminal time
alignment timer is determined to be not running or to be
expired.
[0015] In an aspect of the invention, the step of ignoring the
timing advance command includes storing the time alignment data
received via the random access response.
[0016] In an aspect of the invention, the method includes applying
the stored time alignment data if the mobile communication terminal
time alignment timer expires before contention resolution is
completed.
[0017] In an aspect of the invention, the method includes
discarding the time alignment information and not stopping the
mobile communication terminal time alignment timer if contention
resolution fails.
[0018] In an aspect of the invention, the step of not stopping the
time alignment timer includes maintaining previously assigned
resources.
[0019] In an aspect of the invention, the step of maintaining
previously assigned resources includes maintaining a physical
uplink control channel (PUCCH), sounding reference symbols (SRS),
any configured downlink assignments and uplink grants.
[0020] In an aspect of the invention, there is a mobile
communication terminal configured to perform a contention based
random access procedure with a base station. The mobile
communication terminal includes a transceiver; a display; and a
processor operatively connected to the transceiver and display, the
processor including a time alignment timer. The processor is
configured to transmit a random access preamble message to the base
station; receive a random access response from the base station,
the random access response including a timing advance command;
determine a status of the time alignment timer; and ignore the
timing advance command if the mobile communication terminal time
alignment timer is determined to be running.
[0021] In an aspect of the invention, the timing advance command is
configured to adjust, start or restart the mobile communication
terminal time alignment timer.
[0022] In an aspect of the invention, the processor is configured
to initiate contention resolution by transmitting a scheduled
message to the base station.
[0023] In an aspect of the invention, the processor is configured
to apply the timing advance command to the time alignment timer if
the time alignment timer is determined to be not running or to be
expired.
[0024] In an aspect of the invention, the processor is configured
to store the time alignment data received via the random access
response.
[0025] In an aspect of the invention, the processor is configured
to apply the stored time alignment data if the time alignment timer
expires before contention resolution is completed.
[0026] In an aspect of the invention, the processor is configured
to discard the time alignment information and not stop the time
alignment timer if contention resolution fails.
[0027] In an aspect of the invention, the processor is configured
to maintain previously assigned resources if the time alignment
timer is not stopped.
[0028] In an aspect of the invention, the processor is configured
to maintain a physical uplink control channel (PUCCH) sounding
reference symbols (SRS), any previously configured downlink
assignments and uplink grants, if the time alignment timer is not
stopped.
[0029] In an aspect of the invention, there is a method of
performing a contention based random access procedure by a mobile
communication terminal in communication with a base station. The
method includes transmitting a random access preamble message to
the base station; receiving a random access response from the base
station, the random access response including a timing advance
command; determining a status of a mobile communication terminal
time alignment timer; and if the mobile communication terminal time
alignment timer is determined to be running in the determining
step, ignoring the timing advance command, and transmitting a
scheduled message to the base station in correspondence with the
running mobile communication terminal time alignment timer.
[0030] In an aspect of the invention, if the mobile communication
terminal time alignment timer is determined to be not running or to
be expired, the method includes applying the timing advance command
to restart the mobile communication terminal time alignment timer,
and transmitting a scheduled message to the base station in
correspondence with the restarted mobile communication terminal
time alignment timer.
[0031] Even if a user equipment fails in contention resolution in a
random access procedure, it is possible to prevent a case where
uplink transmission fails due to incorrect uplink synchronization,
which leads to interference to another equipment.
DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 shows an example of a structure of a wireless
communication system according to an embodiment of the
invention.
[0033] FIG. 2 is a diagram showing an example of functional split
between an evolved universal terrestrial radio access network
(E-UTRAN) and an evolved packet core (EPC) according to an
embodiment of the invention.
[0034] FIG. 3 is a block diagram showing constitutional elements of
an example of a user equipment according to an embodiment of the
invention.
[0035] FIG. 4 is a diagram showing an example of a radio protocol
architecture for a user plane according to an embodiment of the
invention.
[0036] FIG. 5 is a diagram showing an example of a radio protocol
architecture for a control plane according to an embodiment of the
invention.
[0037] FIG. 6 shows an example of a structure of a subframe
according to an embodiment of the invention.
[0038] FIG. 7 is a flow diagram showing a related art random access
procedure.
[0039] FIG. 8 is a flow diagram showing a random access procedure
according to an embodiment of the present invention.
[0040] FIG. 9 is a flow diagram showing a random access procedure
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0041] FIG. 1 shows an example of a structure of a wireless
communication system according to an embodiment of the invention.
The wireless communication system may have a network structure of
an evolved-universal mobile telecommunications system (E-UMTS). The
E-UMTS may also be referred to as a long term evolution (LTE)
system. The wireless communication system can be widely deployed to
provide a variety of communication services, such as voices, packet
data, etc.
[0042] Referring to FIG. 1, an evolved-UMTS terrestrial radio
access network (E-UTRAN) includes at least one base station (BS) 20
which provides a control plane and a user plane.
[0043] A user equipment (UE) 10 may be fixed or mobile, and may be
referred to as another terminology, such as a mobile station (MS),
a user terminal (UT), a subscriber station (SS), a wireless device,
etc. The BS 20 is generally a fixed station that communicates with
the UE 10 and may be referred to as another terminology, such as an
evolved node-B (eNB), a base transceiver system (BTS), an access
point, etc. There are one or more cells within the coverage of the
BS 20. Interfaces for transmitting user traffic or control traffic
may be used between the BSs 20. Hereinafter, a downlink is defined
as a communication link from the BS 20 to the UE 10, and an uplink
is defined as a communication link from the UE 10 to the BS 20.
[0044] The BSs 20 are interconnected by means of an X2 interface.
The BSs 20 are also connected by means of an S1 interface to an
evolved packet core (EPC), more specifically, to a mobility
management entity (MME)/serving gateway (S-GW) 30. The S1 interface
supports a many-to-many relation between the BS 20 and the MME/S-GW
30.
[0045] FIG. 2 is a diagram showing an example of a functional split
between the E-UTRAN and the EPC according to an embodiment of the
invention.
[0046] Referring to FIG. 2, slashed boxes depict radio protocol
layers and white boxes depict functional entities of the control
plane.
[0047] The BS performs the following functions: (1) functions for
radio resource management (RRM) such as radio bearer control, radio
admission control, connection mobility control, and dynamic
allocation of resources to the UE; (2) Internet protocol (IP)
header compression and encryption of user data streams; (3) routing
of user plane data to the S-GW; (4) scheduling and transmission of
paging messages; (5) scheduling and transmission of broadcast
information; and (6) measurement and measurement reporting
configuration for mobility and scheduling.
[0048] The MME performs the following functions: (1) distribution
of paging messages to BSs; (2) security control; (3) idle state
mobility control; (4) system architecture evolution (SAE) bearer
control; and (5) ciphering and integrity protection of non-access
stratum (NAS) signaling.
[0049] The S-GW performs the following functions: (1) termination
of user plane packet for paging; and (2) user plane switching for
the support of UE mobility.
[0050] FIG. 3 is a block diagram showing constitutional elements of
an example of a UE according to an embodiment of the invention and
configured to perform the methods shown in FIGS. 8-9. A UE 50
includes a processor 51, a memory 52, a transceiver 53, a display
54, and a user interface unit 55. Layers of the radio interface
protocol are implemented in the processor 51. The processor 51
provides the control plane and the user plane. The function of each
layer can be implemented in the processor 51. The memory 52 is
coupled to the processor 51 and stores an operating system,
applications, and general files. The display 54 displays a variety
of information of the UE 50 and may use a well-known element such
as a liquid crystal display (LCD), an organic light emitting diode
(OLED), etc. The user interface unit 55 can be configured with a
combination of well-known user interfaces such as a keypad, a touch
screen, etc. The transceiver 53 is coupled to the processor 51 and
transmits and/or receives radio signals.
[0051] Layers of a radio interface protocol between the UE and the
network can be classified into a first layer (L1), a second layer
(L2), and a third layer (L3) based on the lower three layers of the
open system interconnection (OSI) model that is well-known in the
communication system. A physical layer, or simply a PHY layer,
belongs to the first layer and provides an information transfer
service through a physical channel. A radio resource control (RRC)
layer belongs to the third layer and serves to control radio
resources between the UE and the network. The UE and the network
exchange RRC messages via the RRC layer.
[0052] FIG. 4 is a diagram showing an example of a radio protocol
architecture for the user plane according to an embodiment of the
invention. FIG. 5 is a diagram showing an example of a radio
protocol architecture for the control plane according to an
embodiment of the invention. They illustrate the architecture of a
radio interface protocol between the UE and the E-UTRAN. The user
plane is a protocol stack for user data transmission. The control
plane is a protocol stack for control signal transmission.
[0053] Referring to FIGS. 4 and 5, a PHY layer belongs to the first
layer and provides an upper layer with an information transfer
service through a physical channel. The PHY layer is coupled with a
medium access control (MAC) layer, i.e., an upper layer of the PHY
layer, through a transport channel. Data is transferred between the
MAC layer and the PHY layer through the transport channel. Between
different PHY layers (i.e., a PHY layer of a transmitter and a PHY
layer of a receiver), data is transferred through the physical
channel. In the PHY layer, modulation is performed using an
orthogonal frequency division multiplexing (OFDM) scheme and time
and frequency can be utilized as a radio resource.
[0054] The MAC layer belongs to the second layer and provides
services to a radio link control (RLC) layer, i.e., an upper layer
of the MAC layer, through a logical channel. The RLC layer in the
second layer supports reliable data transfer. There are three
operating modes in the RLC layer, that is, a transparent mode (TM),
an unacknowledged mode (UM), and an acknowledged mode (AM)
according to a data transfer method. An AM RLC provides
bidirectional data transmission services and supports
retransmission when the transfer of the RLC protocol data unit
(PDU) fails.
[0055] A packet data convergence protocol (PDCP) belonging to the
second layer performs header compression function. When
transmitting an Internet protocol (IP) packet such as an IPv4
packet or an IPv6 packet, the header of the IP packet may contain
relatively large and unnecessary control information. The PDCP
layer reduces the header size of the IP packet so as to efficiently
transmit the IP packet.
[0056] A radio resource control (RRC) layer belongs to the third
layer and is defined only in the control plane. The RRC layer
serves to control the logical channel, the transport channel, and
the physical channel in association with configuration,
reconfiguration and release of radio bearers (RBs). An RB is a
service provided by the second layer for data transmission between
the UE and the E-UTRAN. When an RRC connection is established
between an RRC layer of the UE and an RRC layer of the network, it
is called that the UE is in an RRC connected mode. When the RRC
connection is not established yet, it is called that the UE is in
an RRC idle mode.
[0057] A non-access stratum (NAS) layer belongs to an upper layer
of the RRC layer and serves to perform session management, mobility
management, or the like.
[0058] Data is transmitted from the network to the UE through a
downlink transport channel. Examples of the downlink transport
channel include a broadcast channel (BCH) for transmitting system
information and a downlink-shared channel (DL-SCH) for transmitting
user traffic or control messages. User traffic of downlink
multicast or broadcast service or control messages can be
transmitted on the DL-SCH or a downlink multicast channel (MCH).
Data is transmitted from the UE to the network through an uplink
transport channel. Examples of the uplink transport channel include
a random access channel (RACH) for transmitting an initial control
message and an uplink-shared channel (UL-SCH) for transmitting user
traffic or control message.
[0059] Downlink physical channels are mapped to the downlink
transport channels. Examples of the downlink physical channels
include a physical broadcast channel (PBCH) mapped to the BCH, a
physical multicast channel (PMCH) mapped to the MCH, a physical
downlink shared channel (PDSCH) mapped to the PCH and the DL-SCH,
and a physical downlink control channel (PDCCH) for transmitting
control information (e.g., downlink (DL)/uplink (UL) scheduling
grant, etc.,) which is provided from the first layer and the second
layer. The PDCCH is also referred to as a downlink L1/L2 control
channel. Uplink physical channels are mapped to the uplink
transport channels. Examples of the uplink physical channels
include a physical uplink shared channel (PUSCH) mapped to the
UL-SCH, a physical random access channel (PRACH) mapped to the
RACH, and a physical uplink control channel (PUCCH) for
transmitting control information (e.g., hybrid automatic repeat
request (HARD) acknowledgment (ACK)/negative-ACK (HACK) signals, a
scheduling request signal, a channel quality indicator (CQI),
etc.,) which is provided from the first layer and the second
layer.
[0060] FIG. 6 shows an example of a structure of a subframe
according to an embodiment of the invention.
[0061] Referring to FIG. 6, a subframe includes a plurality of OFDM
symbols and a plurality of subcarriers. The subframe is a basic
unit of radio resource allocation. One subframe consists of a
plurality of resource blocks. One resource block includes a
plurality of subcarriers (e.g., 12 subcarriers). The subframe can
be divided into a region where a physical downlink control channel
(PDCCH) (also referred to as an L1/L2 control channel) is
transmitted and a region where a physical downlink shared channel
(PDSCH) is transmitted. For example, the PDCCH may correspond to
first three OFDM symbols in the subframe. A time for transmitting
one subframe is referred to as a transmission time interval (TTI).
For example, 1 TTI may be 1 millisecond (ms). One subframe can be
divided into two slots in the time domain. If 1 TTI=1 ms, one slot
has a length of 0.5 ms.
[0062] FIG. 7 is a flow diagram showing a related art random access
procedure.
[0063] The random access procedure may be performed for the
following purposes: (1) an initial access process; (2) a handover
process; (3) a process of transmitting downlink data to a UE that
is not time synchronized; (4) a process of transmitting data in
uplink by the UE that is not time synchronized; and (5) a recovery
process performed when an error occurs in a wireless
connection.
[0064] Referring to FIG. 7, a UE randomly selects a random access
preamble from a plurality of available random access preambles. The
information to generate the plurality of available random access
preambles can be received as a part of system information. The UE
transmits the randomly selected random access preamble to the BS by
using a PRACH resource (step 710).
[0065] Upon receiving the random access preamble from the UE, the
BS transmits a random access response to the UE through a DL-SCH
(step 720). The random access response may include time alignment
information (i.e., a time advance value or timing advance) for
correcting uplink time alignment, uplink radio resource allocation,
an index of the random access preamble (i.e., preamble identifier
(Id)), a temporary cell-radio network temporary identifier
(C-RNTI), etc. The random access response on the DL-SCH can be
indicated by a PDCCH addressed by a random access identifier. The
random access identifier is also referred to as a random
access-radio network temporary identifier (RA-RNTI).
[0066] After receiving random access response, the UE can correct
time alignment based on the time alignment information. However, in
an embodiment of the present invention, instead of correcting time
alignment immediately after the UE receives the random access
response, time alignment information may be temporarily stored so
that time alignment can be corrected by using the time alignment
information when a certain condition (i.e., contention resolution,
etc.) is satisfied. This process will be described below in greater
detail with reference to FIG. 8.
[0067] The UE transmits a scheduled message to the BS through a
UL-SCH by using the uplink radio resource allocation in the random
access response (step 730).
[0068] Upon receiving the scheduled message, the BS transmits a
contention resolution message to the UE (step 740).
[0069] Now, contention resolution performed in a random access
procedure will be described in detail.
[0070] Collision occurs in the random access process because the
number of available random access preambles is limited while a
large number of UEs are used. That is, it is difficult for the BS
to provide respective dedicated random access preambles to all UEs.
Therefore, one UE needs to randomly select and transmit one of
random access preambles provided also to other UEs. Accordingly, in
some cases, two or more UEs may select and transmit the same random
access preamble by using the same PRACH resource. This is called a
contention state.
[0071] The BS regards two or more identical random access preambles
transmitted from two or more UEs as one random access preamble
transmitted from one UE. Further, the BS transmits to the UEs the
same random access response for the received random access
preamble.
[0072] When collision occurs, the same random access response is
received by the two or more UEs, and the respective UEs perform
different operations according to one random access response. That
is, by using the uplink radio resource allocation in the random
access response, the UEs transmit different scheduled messages.
Accordingly, the UEs may fail to transmit the scheduled message, or
only a specific UE may succeed in transmitting the scheduled
message according to a location or a transmit power of each UE.
[0073] In the latter case where only the specific UE succeeds in
transmitting the scheduled message, two or more UEs determine that
they succeed in data transmission. Therefore, the BS has to inform
the UEs, which have failed in contention, of the fact that data
transmission has failed. That is, contention resolution is an
operation of informing a UE of whether contention caused by
collision is successful or unsuccessful when collision occurs
between UEs in the random access procedure.
[0074] There are two conventional contention resolution methods,
that is, a first method of using a contention resolution timer and
a second method of transmitting an identifier of a UE which
succeeds in contention.
[0075] In the first conventional method, it is required that the UE
already has a unique cell identifier (e.g., C-RNTI) before a random
access procedure is performed. The UE which already has its cell
identifier transmits data including the cell identifier to the BS
according to a random access response message, and then starts the
contention resolution timer. If the UE receives the data including
the cell identifier through a PDCCH before the contention
resolution timer expires, the UE determines that the UE succeeds in
contention and thus successfully finishes the random access
procedure.
[0076] On the contrary, in the first method, if the UE fails to
receive the data including the cell identifier through the PDCCH
before the contention resolution timer expires, the UE determines
that the UE fails in contention. Then, the UE may re-perform the
random access procedure or may provide a failure report to an upper
layer.
[0077] The second conventional method of the two contention
resolution methods is used when the UE does not have its unique
cell identifier before the random access procedure is performed.
That is, if the UE does not have its cell identifier, the UE
transmits data including an S-temporary mobile subscriber identity
(S-TMSI) or a random Id which is a higher-level identifier than a
cell identifier according to uplink radio resource allocation in
the random access response, and then starts the contention
resolution timer.
[0078] If the data including the higher-level identifier is
transmitted through a DL-SCH before the contention resolution timer
expires, the UE determines that the random access procedure is
successful. On the contrary, if the UE fails to receive the data
including the higher-level identifier through the DL-SCH before the
contention resolution timer expires, the UE determines that the
random access procedure is unsuccessful.
[0079] Now, time alignment and time alignment correction will be
described. The random access procedure is one of methods for uplink
time alignment.
[0080] When collision does not occur, a time alignment correction
method is performed according to the random access procedure as
follows. A BS can measure a time alignment value by using a random
access preamble transmitted from a UE, and provide time alignment
information for time alignment correction to the UE. Upon receiving
the random access response, the UE applies the received time
alignment information, and starts or restarts a time alignment
timer.
[0081] It is assumed that time alignment is maintained between the
UE and the BS while the time alignment timer is running. Further,
it is also assumed that time alignment is not maintained between
the UE and the BS when the timer alignment timer expires. The BS
can measure a time alignment value of the DE by using another
method other than the method of using the random access preamble.
The BS can optionally provide the time alignment information to the
UE.
[0082] Upon receiving the time alignment information, the UE
applies the received time alignment information, and starts or
restarts the time alignment timer. When the time alignment timer
expires, it is prohibited for the UE to perform uplink transmission
except for transmission of the random access preamble.
[0083] In the random access procedure, there already exists a
possibility of collision. As a result, incorrect time alignment
information may be applied to the UE due to collision. That is, if
time alignment is achieved before the UE transmits the random
access preamble, incorrect uplink transmission may be caused due to
collision. Accordingly, time alignment is corrected using incorrect
time alignment information.
[0084] In the following description, it is assumed that uplink time
alignment is achieved between the UE and the BS. In this case, time
alignment timers used in the UE and the BS are synchronized with
each other. It is also assumed that uplink data transmission is
requested in a state where the UE does not have an uplink radio
resource, and thus the UE performs a random access procedure.
[0085] The UE transmits a random access preamble to the BS, and
receives a random access response. The UE applies time alignment
information included in the received random access response to the
UE itself, and starts the time alignment timer.
[0086] If collision occurs and thus the received random access
response is a random access response to be transmitted to another
UE, the UE may receive incorrect time alignment information.
However, since the time alignment information included in the
random access response is applied to the UE and thus the time
alignment timer is running in the UE, the UE determines that time
alignment is achieved between the UE and the BS. Therefore, if data
reception is requested after the UE receives random access response
information, the UE attempts uplink transmission according to the
incorrect time alignment, resulting in interference to transmission
of other UEs.
[0087] Accordingly, when the random access procedure is performed
by the UE which is time-aligned with the BS, the time alignment
information included in the random access response is applied after
contention resolution is completed. That is, the UE applies the
time alignment information after collision occurs, and then can
start or restart the time alignment timer.
[0088] FIG. 8 is a flow diagram showing a random access procedure
according to an embodiment of the present invention. This is a case
where a UE performing the random access procedure succeeds in
contention caused by collision.
[0089] Referring to FIG. 8, the UE operates in a state where uplink
time alignment is achieved between the UE and a BS and a time
alignment timer is running (step 810). That is, the UE has a valid
time alignment value, and the time alignment timer is running in
the UE.
[0090] When uplink data transmission is requested in a situation
where there is no uplink radio resource, the UE starts a contention
based random access procedure. Accordingly, the UE transmits a
selected random access preamble to the BS (step 820). The BS
receives the random access preamble from the UE and then transmits
a random access response to the UE in response thereto (step 830).
The random access response may include radio resource allocation
for transmitting a scheduled message, a preamble identifier, time
alignment information (e.g., a timing advance command), a temporary
C-RNTI, etc.
[0091] If, upon receipt of the time alignment information, the UE's
time alignment timer is determined to be running, the UE neither
applies the received time alignment information nor starts the time
alignment timer (e.g., until contention resolution is completed).
Instead received time alignment information may be stored in a
buffer and the UE maintains a previous time alignment value used
before the random access procedure is performed and also maintains
an operation of the time alignment timer which is currently
running. In other words, the UE ignores the received time alignment
information and maintains a previous time alignment value used
before the random access procedure is performed and also maintains
an operation of the time alignment timer that is currently running.
The UE may apply the stored time alignment data if the mobile
communication terminal time alignment timer expires before
contention resolution is completed.
[0092] The UE transmits to the BS the scheduled message including
an identifier (e.g., C-RNTI, S-TMSI, random Id, etc.) by using the
radio resource allocation information (i.e., UL grant information)
included in the received random access response (step 840).
Thereafter, the UE receives a contention resolution message before
a contention resolution timer included in the UE expires (step
850).
[0093] If the UE receives a decodable contention resolution message
before the contention resolution timer expires, the UE determines
that the UE succeeds in contention caused by collision. That is, if
the received contention resolution message is a PDCCH including a
cell identifier of the UE or a DL-SCH including a higher-level
identifier (S-TMSI or random Id) of the UE, the UE determines the
UE succeeds in contention.
[0094] If, after receipt of the time alignment information, the
UE's time alignment timer is determined to be not running or to be
expired, the UE applies time alignment information obtained from
random access information to a previous time alignment value. Then,
the DE restarts the time alignment timer (step 860).
[0095] FIG. 9 is a flow diagram showing a random access procedure
according to an embodiment of the present invention. This is a case
where a UE performing the random access procedure fails in
contention caused by collision.
[0096] Referring to FIG. 9, the UE operates in a state where uplink
time alignment is achieved between the UE and a BS and a time
alignment timer is running (step 910). The UE transmits a selected
random access preamble to the BS (step 920). The BS receives the
random access preamble from the UE and then transmits a random
access response to the UE in response thereto (step 930). The UE
neither applies the received time alignment information nor starts
the time alignment timer until contention resolution is completed.
The received time alignment information is stored in a buffer. The
UE maintains a previous time alignment value used before the random
access procedure is performed and also maintains an operation of
the time alignment timer which is currently running. The UE
transmits to the BS a scheduled message including an identifier by
using the radio resource allocation information (i.e., UL grant
information) included in the received random access response (step
940).
[0097] After receiving the scheduled message, the UE starts a
contention resolution timer (step 950). Unlike the embodiment of
FIG. 8, the UE cannot receive its contention resolution message
from the BS after the contention resolution timer starts until the
contention resolution timer expires. For example, a cell identifier
or a higher-level identifier of the UE may not be included in the
contention message, or the contention message including the cell
identifier or the higher-level identifier of the UE may not be
received by the UE before the contention resolution timer expires.
In this case, the UE determines that the UE fails in
contention.
[0098] When the contention resolution timer expires, the UE does
not apply the received time alignment information but discards the
time alignment information (step 960) but does not stop the time
alignment timer. Accordingly, a previous operation of the time
alignment timer is maintained without alteration. Thus, previously
assigned resources (e.g., PUCCH, sounding reference symbols (SRS),
any previously configured downlink assignments and uplink grants)
are maintained.
[0099] Because the received time alignment information is not
applied when the UE fails in contention, transmission error or
interference can be reduced when uplink synchronization is
incorrectly achieved.
[0100] The methods of FIGS. 8-9 may be performed in a device
similar the one depicted in FIG. 3, or in another device. The
present invention can be implemented with hardware, software, or
combination thereof. In hardware implementation, the present
invention can be implemented with one of an application specific
integrated circuit (ASIC), a digital signal processor (DSP), a
programmable logic device (PLD), a field programmable gate array
(FPGA), a processor, a controller, a microprocessor, other
electronic units, and combination thereof, which are designed to
perform the aforementioned functions. In software implementation,
the present invention can be implemented with a module for
performing the aforementioned functions. Software is storable in a
memory unit and executed by the processor. Various means widely
known to those skilled in the art can be used as the memory unit or
the processor.
[0101] While the present invention has been particularly shown and
described with reference to exemplary embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims. The exemplary embodiments should be considered in
descriptive sense only and not for purposes of limitation.
Therefore, the scope of the invention is defined not by the
detailed description of the invention but by the appended claims,
and all differences within the scope will be construed as being
included in the present invention.
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